So I started working on an example for the example browser of Bullet Physics and I would like to get the Newtonian pendulum (Newton's Cradle) to work properly. However, if one pendulum hits the 4 others, they all somehow move a bit and not only the one on the other side. I am using restitution to get the pendula fully elastic (max realistic restitution is 1, right?), also I found out that there is a hitFraction, but it does not do anything visible. Any idea on how to fix this? I appended my example files. You can move the pendulum using key 3, and change the lengths of the pendula using key 1 and 2.
Here is a video of what I see with 5 and 3 pendula:
https://youtu.be/leDccYvnRzU
[SOLVED] Newton's cradle not transforming force properly
[SOLVED] Newton's cradle not transforming force properly
 Attachments

 NewtonianPendulum.h
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 NewtonianPendulum.cpp
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Last edited by benelot on Fri Jun 03, 2016 3:37 pm, edited 1 time in total.
Re: Newtonian pendulum not transforming force properly
Took a look at the code and video but haven't added it to my project yet so can't say the following is 100% accurate/going to fix it, but:
Video looks like it's doing what it's supposed to, at least from a theoretical perspective. In one step there's zero motion so zero propagation. Even in realworld the momentum transfer isn't instantaneous. The multibody method might work a bit better, but not sure if the setup would be as trivial
From the above, there's currently no room for motion, so maybe adding a bit of empty space will help. Propagation will end up taking n frames (n=pendulums2), but it will likely be a bit more visually correct
 Similarly, you can try using a smaller timestep and more rigid solver, or even multibody
I might try it out in the morning after tuning my example a bit (need the right defaults to show off the methods).
Video looks like it's doing what it's supposed to, at least from a theoretical perspective. In one step there's zero motion so zero propagation. Even in realworld the momentum transfer isn't instantaneous. The multibody method might work a bit better, but not sure if the setup would be as trivial
From the above, there's currently no room for motion, so maybe adding a bit of empty space will help. Propagation will end up taking n frames (n=pendulums2), but it will likely be a bit more visually correct
 Similarly, you can try using a smaller timestep and more rigid solver, or even multibody
I might try it out in the morning after tuning my example a bit (need the right defaults to show off the methods).
Re: Newtonian pendulum not transforming force properly
Hello Basroil,
Thanks for posting, small spaces between the balls indeed make it slightly more realistic. Still it does not make it move in a perfect force transformation manner (https://www.youtube.com/watch?v=JadO3RuOJGU), but who cares. It is well enough for an example. I will quickly try different solvers and a smaller timestep, maybe it helps.
Edit: The problem was stupidly simple, I forgot to set the intial restitution to 1. So the force did not propagate, only after a slight move of the restitution slider, which set the restitution for the first time.
Thanks for posting, small spaces between the balls indeed make it slightly more realistic. Still it does not make it move in a perfect force transformation manner (https://www.youtube.com/watch?v=JadO3RuOJGU), but who cares. It is well enough for an example. I will quickly try different solvers and a smaller timestep, maybe it helps.
Edit: The problem was stupidly simple, I forgot to set the intial restitution to 1. So the force did not propagate, only after a slight move of the restitution slider, which set the restitution for the first time.